1. Field of the Invention
The present invention relates to a method for determining a parameter, such as the resistance, the state of charge or the capacity, of at least one accumulator of a battery, notably in order to determine the state of health of the battery; as well as to an electronic system for monitoring a battery.
2. Background
Typically a battery comprises a plurality of accumulators also called electrochemical generators, cells or elements. An accumulator is a device for producing electricity in which chemical energy is converted into electrical energy. The chemical energy is formed by electrochemically active compounds deposited on at least one face of electrodes positioned in the accumulator. Electrical energy is produced by electrochemical reactions during discharge of the accumulator. The electrodes, positioned in a container are electrically connected to current output terminals which ensure electrical continuity between the electrodes and an electricity consuming device with which the accumulator is associated.
A battery may be divided into modules, each module consisting of a plurality of accumulators connected together in series and/or in parallel. The battery is intended to provide electrical energy to an outer application. A charging circuit is generally provided to which the battery may be connected for recharging the accumulators. The battery may include one or more parallel branches of accumulators connected in series and/or one or more parallel branches of modules connected in series. A management system comprising measurement sensors and an electronic control circuit, which is more or less sophisticated depending on the applications, may be associated with the battery.
The state of charge (SOC) and the state of health (SOH) are pieces of information useful for the electronic management system of the battery in order to optimize its use and its lifetime.
Typically, the state of charge SOC is determined as the amount of energy available in the battery, relatively to the energy of a totally charged battery. The state of charge SOC may be calculated for example according to an algorithm using voltage measurements or integration of the charging/discharging current over time depending on the current conditions of the battery.
The state of health SOH of the battery allows an estimation of the ageing of the battery between a new state and an end-of-life state.
A first method for determining the SOH of a battery, a so-called static method, consists of monitoring the values of temperature, of voltage and optionally of current of the battery in order to determine an SOH value from ageing laws. These ageing laws are obtained from tests conducted in a laboratory. An SOH algorithm then gives an estimation of the ageing of the battery. However this method for determining the SOH of the battery is subject to the assumption of homogeneous ageing of the accumulators of the battery. The method for determining the SOH by a predictive model also assumes a flawless power circuit between the accumulators.
A second method for determining the SOH of a battery, a so-called dynamic method, consists of calculating the ratio of the resistance of the battery at a given instant over the resistance of the battery in the new condition under the same measurement conditions. The SOH may also be calculated from the ratio of the capacity of the battery at a given instant over the capacity of the battery in the new condition. Depending on the size of the battery and/or depending on the applications, a calculation of SOH may be provided for the battery as a whole or for each module or for each accumulator.
Typically, the resistance of a battery (or of an accumulator of the battery) is measured according to a voltage drop over only two points, while considering a relative uncertainty related to the measurement determined by a conventional uncertainty calculation method.
FIG. 1 illustrates ideal simulated current and voltage signals of a battery during a current pulse. A determination of the resistance may be carried out during such a pulse: a first voltage measurement U1 is conducted when the current is 0, the voltage then being 3.944 V; and a second voltage measurement U2 is conducted during the pulse with a current of −60 A, the voltage being 3.864 V. A resistance value R is then obtained, determined as follows:R=ΔV/ΔI=1.33 mΩ
However, in reality, whether the pulse is caused during a maintenance cycle or on an onboard application, the ideal signals illustrated in FIG. 1 do not exist but have perturbations which take into account inter alia the uncertainties of the voltage and current sensors. FIG. 2 illustrates noisy simulated current and voltage signals of a battery during a current pulse. Typically, the current sensor has a 5% ripple in the low portion of the pulse and the voltage sensor has a ripple related to the uncertainty of 20 mV. Thus, with a relative uncertainty of 5% on the current and an absolute uncertainty of 20 mV on the voltage, the relative uncertainty on the resistance measurement is calculated as follows by using a conventional method for calculating uncertainties:dR/R=dΔV/ΔV+dΔI/ΔI=dU1/ΔV+dU2/ΔV+dI1/ΔI+dI2/ΔI, 
i.e. an uncertainty of 60%.
Also, if the uncertainty is calculated by using the method of quadratic sums:dR/R=√(dΔV/ΔV)2+(dΔVΔI)2,
an uncertainty value of 51% is obtained which remains far from a reliable value.
Moreover, the resistance measurement requires a current pulse in order to cause a voltage drop allowing the measurement of two pairs of voltage and current values. For this purpose, the resistance of the battery (or of an accumulator of the battery) is generally measured during a maintenance operation; for example upon checking the connection of the battery.
When the battery is connected to an application, it is possible to estimate the state of health by means of predictive models. For example, document FR-A-2 920 884 describes a method for estimating the state of health of a battery onboard an automobile. This method detects a stable state of the battery and generates a current at its terminals in order to estimate the value of the internal resistance from changes in current and voltage. The value of the internal resistance is then related to an estimation of the state of health of the battery by means of mapping.
Document WO-A-2007/004817 proposes determination of the internal resistance of the battery from measurements of temperature and from estimations of the state of charge; an instantaneous resistance is measured from a pair of voltage-current values and then normalized. The value of the internal resistance is then related to an estimation of the state of health of the battery by means of a table.
Document US-A-2009/0140744 proposes determination of a pseudo-impedance of the battery from discharge pulses imposed to the battery. The state of health of the battery is then determined from this pseudo-impedance.
Document US-A-2009/0085517 describes a method for managing the charging of a battery in a portable appliance which uses a statistical method for comparing the value of the remaining capacity with the initial value.
Methods known for determining the resistance of a battery (or of an accumulator of a battery) use estimations and/or direct measurements accompanied by significant uncertainty. The subsequent determination of the state of health of the battery is strongly affected by this.
Therefore, there exists a need for a method for determining the resistance of at least one accumulator of a battery which has reduced and known uncertainty in order to increase the confidence related to the resistance value and therefore to the subsequent determination of a state of health. There also exists a need for a method for determining the resistance of at least one accumulator of a battery which may be applied independently when the battery is being used or in maintenance.
Moreover, as indicated above, the state of health may be determined from the capacity of at least one accumulator of the battery.
In a way known per se, the capacity of an accumulator may be calculated from values of the state of charge at different instants, notably for accumulators of the Li-ion type which have a direct relationship between the SOC and the no-load voltage. FIG. 12 shows such a curve of the state of charge versus the no-load voltage for a Li-ion accumulator.
The capacity of an accumulator is expressed in Ah by the following relationship:
  C  =                    ∫                  t          initial                          t          final                    ⁢              I        ⁢                              ⅆ            t                    /          3600                                    SOC        ⁡                  (                      t            final                    )                    -              SOC        ⁡                  (                      t            initial                    )                    
The determination of the SOC values at two relevant instants (initial and final instants in the formula above) requires a measurement of current and voltage at these instants. Like for determining the resistance, the uncertainty on the measurement of current and of voltage entails uncertainty on the SOC values and therefore on the value of the capacity. Known methods for determining the capacity of a battery (or of an accumulator of a battery) use estimations and/or direct measurements accompanied by significant uncertainty. The subsequent determination of the state of health of the battery is strongly affected by this.
Therefore, there also exists a need for a method for determining the capacity of at least one accumulator of a battery which has a reduced and known uncertainty in order to increase confidence related to the capacity value and therefore to the subsequent determination of a state of health.